Method for inhibition of growth of organisms on faces of constructions submerged in a liquid

ABSTRACT

The invention concerns a method for inhibition of growth of organisms on faces of constructions (11) submerged in a liquid. In themethod, an electrically conductive structure (11) to be protected is connected as the cathode of a source (14) of direct current, or an electrically non-conductive structure (111) to be protected is first coated with an electrically conductive material (111a) and connected as the cathode of a source of direct current (14), respectively, and, as the anode (12), an anode is used that has been isolated from the structure (11) to be protected or that is placed separate from said structure, which anode is connected as the anode of the source (14) of direct current. A control signal is given to the source (14) of direct current from a control unit (15), which control signal changes the current density and/or the voltage supplied by the source (14) of direct current, whereby the pH of the liquid on the face of the structure (11) to be protected varies with such of a frequency that the microbial organisms on the face of the structure (11) to be protected cannot adapt themselves to the changing conditions.

This application is a 371 of PCT/FI95/00602 filed on Nov. 1, 1995.

BACKGROUND OF THE INVENTION

The invention concerns a method for inhibition of growth of organisms onfaces of constructions submerged in a liquid, in which method anelectrically conductive structure to be protected is connected as thecathode of a source of direct current, or an electrically non-conductivestructure to be protected is first coated with an electricallyconductive material and connected as the cathode of a source of directcurrent, respectively, and, as the anode, an anode is used that has beenisolated from the structure to be protected or that is placed separatefrom said structure, which anode is connected as the anode of the sourceof direct current.

The phenomenon of fouling means covering of faces in contact with waterwith colonies formed by organisms adhering to said faces. Fouling isproduced both by micro-organisms and by plants and animals. Foulingusually starts with adhering and spreading of populations of bacteriaover faces that are in contact with water. The bacteria pioneers arefollowed by numerous different algae and other organisms with genuinenuclei, such as barnacles and polyps.

The fouling phenomenon is perhaps most harmful to waterborne traffic(the fuel consumption may increase by up to 40 percent), to industrialplants and power stations that use seawater, and to fish breedingplants.

In the waters of Finland, the fouling trouble was little in the pastyears. Eutrophication of the water areas near the coasts of the BalticSea and an increase in the salt content have increased the disadvantagescaused by fouling, in particular in the case of industrial plants thatuse seawater.

The biggest problems caused by fouling occur in areas in which the saltcontent in seawater is higher than 5 per mil. In a warm area of seawaterwhich contains salt, fouling is a serious problem for all structurespresent in the seawater and for all industrial plants and power stationsthat use seawater as well as for fishing industry. For example, thenumerous population in Asia lives mainly on seafood. Ships cannot leavethe ports before mechanical cleaning of propellers and other controldevices has been carried out.

In order to prevent drawbacks of fouling, at present mainly so-calledanti-fouling paint is used. From the anti-fouling paint, one or severalsubstances toxic to the organisms adhering to the structures areseparated, such as, for example, copper and tin compounds. In additionto the toxic agents, the smooth face of the paint makes the adhering ofthe organisms more difficult. However, the anti-fouling paint must berenewed, on the average, at intervals of two years. Organic tincompounds are efficient in combatting the fouling organisms onunderwater structures, but they are also toxic for other groups oforganisms, such as fish and mammals. Moreover, TBT (tributyl tin) is apoison that accumulates in organisms to a great extent.

Plants and animals can accumulate copper present in dissolved form to acertain extent. Accumulation of copper in the food chain is not known atpresent, but if high concentrations of dissolved copper are present inwater, it may be dangerous to the organisms in the water.

With respect to the prior art, reference is made to the PatentGB-2,118,972, in which the anti-fouling effect described is based onsacrificial Cu/Al or Fe rods. In this prior-art method, Cu/Al or Fe rodsare dissolved by means of direct current, and the system of seawaterpipes or equivalent that constitutes the structure operates as thecathode. For example, the copper-aluminum hydroxide that is formedprevents formation of growth.

In the method described in the publication EP-0,145,802, theanti-fouling effect is produced by means of sacrificial metal plates,most commonly by means of Cu plates. In this method, the structures tobe protected are coated with an insulating layer, onto which a metalplate of a certain size is attached, the size depending on the length ofthe ship. The protection against corrosion of the structures is effectedby supplying a DC-voltage to the hull while graphite, cast iron,platinum-coated titanium, or a Pb/Ag-alloy operates as the anode. Thesource of DC-voltage consists of a potentiostat, which automaticallymaintains the potential of the structure to be protected at the pre-setprotection potential. The copper hydroxide that is dissolved preventsformation of growth.

In the method described in the publication U.S. Pat. No. 5,009,757, aparticular inner Ti electrode and a source of current are employed, anda high capacitance is produced between a zinc coating and the seawater.The zinc-painted hull of the ship operates as the negative terminal ofthe capacitor. The anti-fouling effect is based on the Helmholtz doublelayer produced by the electric current between the zinc coating and theseawater.

The anti-fouling effect described in the Pat. Appl. FI-915300 is basedon ultrasound. The low-frequency oscillations of the sources ofultrasound make the micro-organisms to be separated from the face of thestructure.

In the publication EP-0,468,739, a direct-current method is described,in which an electric shock is given to the microbes growing on the facesto be protected by means of an electric field produced between separateelectrodes. In this method, the structure to be protected is notconnected to the source of current, but the electric current is passedthrough a separate displaceable anode to a separate displaceablecathode.

In the publication EP-0,369,557, a direct-current method is described,in which the structure to be protected is coated with a conductivelayer, on whose face, in an electrolysis of seawater, an anode reactionforms hypochlorite which kills microbes.

In the publication WO-87/03261, a method based on the use of alternatingcurrent is described. In this method, the organisms are destroyed bymeans of an electric shock produced by means of the field of alternatingcurrent. The effect can be intensified by dissolving copper, aluminumand by electrolyzing seawater by means of direct current, in whichconnection the chlorine gas that is formed kills microbes.

The prior-art methods involve a number of drawbacks. When anti-foulingpaints are used, damage to the environment constitutes the majordrawback. Also, the annual cost of maintenance becomes relatively high.Moreover, the anodes that are consumed on dissolution of copper,aluminum and iron cause a need of maintenance.

In the ultrasound method, the most important drawbacks are the high costof the method and the detrimental effects of resonance.

The prior-art electrical methods also involve a number of significantdrawbacks. In cases in which the object to be protected is subjected toan external electric field (direct or alternating current), separateelectrodes that supply current are needed. Also, in these prior-artmethods, a control system that optimizes the current is missing. Anexcessively high current density produces the risk of hydrogenbrittleness in electrically conductive structures. Oxidation, i.e. wear,of a paint that operates as an anode is a clear drawback.

In a method that makes use of the Helmholtz double layer, precipitationof calcium and magnesium on the face and, consequently, formation of aface favourable for growth, is the most important drawback.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improvement overthe prior-art methods and to avoid the numerous drawbacks present in theprior-art methods. It is a more specific object of the invention toprovide a method that is suitable for prevention of growth of organismson the faces of electrically conductive structures and so also ofelectrically non-conductive structures submerged in a liquid.

The objectives of the invention are achieved by means of a method, whichis characterized in that a control signal is given to the source ofdirect current from a control unit, which control signal changes thecurrent density and/or the voltage supplied by the source of directcurrent, whereby the pH of the liquid on the face of the structure to beprotected varies with such a frequency that the microbial organisms onthe face of the structure to be protected cannot adapt themselves to thechanging conditions.

In the method in accordance with the present invention, it has beenrealized to change the current density under control, in which case thepH on the face of the structure varies with such a frequency that themicrobial organisms present on the faces of the structures cannot adaptthemselves to the changing conditions by means of mutations or by meansof changes in the cell wall. In the method of the present invention, aso-called pH-pumping by varying the cathode reaction prevents adaptationof the bacteria to the changing conditions. A rapid increase in pH killsbacteria, and variations in pH also contribute to prevention of theformation of a cathodic precipitate. As a consequence of the cathodereaction, the concentration of hydroxide ions on the face of the coatedstructure increases to such an extent that the microbes die. As a resultof this, strains of organisms that have adapted themselves to living indifferent oxygen concentrations die when the oxygen concentrationchanges.

In the method of the invention, the face of an electrically conductivestructure submerged in water is coated with a paint that is porous in acontrolled way, while the porosity is such that the ions necessary forclosing the current circuit can pass through the paint to such an extentthat a cathode reaction takes place. The structure to be protected isconnected as the cathode of the source of direct current, and as theanode, anodes are used that have been isolated from the structure orthat are separate from the structure, and the supply of current to thestructure to be protected is controlled by means of separate referenceelectrodes isolated from the structure, by means of which referenceelectrodes an excessive supply of current to the structure to beprotected is prevented by monitoring its electrochemical potential. Theelectrochemical properties of such a paint face that is porous in acontrolled way are such that precipitation of anions on the face isimpossible.

The method in accordance with the invention can be applied toelectrically conductive structures submerged in a liquid, such as, forexample, steel and aluminum ships and boats, off-shore constructions,supports, and columns of steel, sluice and gate equipments andstructures for various water ducts, various process actuators placed ina water circulation, such as, for example, heat exchangers and tanks.The invention is also suitable for use in electrically non-conductivestructures submerged in a liquid, such as, for example, wooden boats,pier and support constructions of wood or concrete, structures made ofpolymer composites, such as, for example, boats, cooling ducts etc.water ducts made of concrete.

The invention will be described in detail with reference to somepreferred embodiments of the invention illustrated in the figures in theaccompanying drawings, the invention being, however, not supposed to beconfined to said embodiments alone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an equipment for use in the methodin accordance with the invention for inhibition of growth of organismson faces of electrically conductive constructions submerged in a liquid.

FIG. 2 is a schematic illustration of an electrically non-conductivestructure which has been made electrically conductive.

FIG. 3 is a graphic illustration of the effect of a change in thecurrent density on the pH-value.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the equipment in accordance with the invention is denotedgenerally with the reference numeral 10. The equipment 10 includes acathode 11, which is an electrically conductive structure, an anode 12,a reference electrode 13 isolated from the structure 11, a source 14 ofdirect current, and a control logic, i.e. a control unit 15. The anode12 may be an anode isolated from the structure 11 to be protected, or ananode separate from said structure, as is indicated by the dotted line.Further, the equipment 10 may be provided with a bio-organism detector16, which informs on any bio-organisms that may be placed on the face ofthe structure to be protected.

In FIG. 2, an electrically non-conductive structure is denoted with thereference numeral 111. The structure 111 to be protected is coated witha paint 111a, which operates as the cathode.

From FIG. 3 it is seen that a change in the current density, i.e. anincrease in the current density has a raising effect on the pH-value.The control unit 15 gives the source 14 of direct current a controlsignal that changes the current density, whereby the current density maychange regularly or randomly. The time interval of the change in currentdensity depends on the structure 11,111 to be protected, and it can beof an order of, for example, one second to 24 hours or several days.From FIG. 3 it is seen clearly that, when the current density becomeshigher, the cathode reaction becomes more intensive, as a result ofwhich the pH becomes higher and the oxygen content becomes lower. Thesechanges prevent growth of organisms on the faces of the structure 11,111to be protected highly efficiently.

When the method of the present invention is used for seawaterapplications, the maximal value of current density is, as a rule, of anorder of 2.5 A per sq.m, and/or the maximal value of the voltage is ofan order of 1 V . . . 100 V, whereas, in industrial processes, theintensity may be, for example, of an order of 10 A per sq.m, and/or themaximal value of the voltage is of an order of 100 V.

Above, just the solution of principle of the invention has beendescribed, and it is obvious to a person skilled in the art thatnumerous modifications can be made to said solution within the scope ofthe inventive idea described in the accompanying patent claims.

We claim:
 1. A method for inhibition of growth of organisms on faces ofconstructions (11, 111) submerged in a liquid, in which method anelectrically conductive structure (11) to be protected is connected asthe cathode of a source (14) of direct current, or an electricallynon-conductive structure (111) to be protected is first coated with anelectrically conductive material (111a) and connected as the cathode ofa source of direct current (14), respectively, and, as the anode (12),an anode is used that has been isolated from the structure (11,111) tobe protected or that is placed separate from said structure, which anodeis connected as the anode of the source (14) of direct current,characterized in that a control signal is given to the source (14) ofdirect current from a control unit (15), which control signal changesthe current density and/or the voltage supplied by the source (14) ofdirect current, whereby the pH of the liquid on the face of thestructure (11,111) to be protected varies with such a frequency that themicrobial organisms on the face of the structure (11,111) to beprotected cannot adapt themselves to the changing conditions.
 2. Amethod as claimed in claim 1, characterized in that the current densityand/or the voltage is/are changed regularly at certain time intervals.3. A method as claimed in claim 2, characterized in that the timeinterval of the change in current density and/or voltage is in the rangeof 1 second to several days.
 4. A method as claimed in claim 3,characterized in that the time interval of the change in current densityand/or voltage is in the range of 1 second to 24 hours.
 5. A method asclaimed in claim 1, characterized in that the current density and/or thevoltage is/are changed randomly.
 6. A method as claimed in claim 1,characterized in that, when the current density and/or voltage is/areincreased, the cathode reaction becomes more intensive, as a result ofwhich the pH of the liquid becomes higher and the oxygen content becomeslower.
 7. A method as claimed in claim 1, characterized in that theelectrically conductive structure (11) is coated with a porous paintwhile the porosity is such that the ions necessary for closing thecurrent circuit can pass through the porous paint to such an extent thata cathode reaction takes place.
 8. A method as claimed in claim 1,characterized in that the electrically non-conductive structure (111) tobe protected is coated with a paint (111a) that operates as the cathode.9. A method as claimed in claim 1, characterized in that the supply ofcurrent to the structure (11,111) to be protected is controlled by meansof a separate reference electrode (13) isolated from the structure(11,111) to be protected, which reference electrode prevents anexcessive current supply to the structure (11,111) to be protected bymonitoring its electrochemical potential.
 10. A method as claimed inclaim 1, characterized in that any organisms that may be present on theface of the structure (11,111) to be protected is monitored by means ofa bio-organism detector (16), which gives a signal to the control unit(15).
 11. A method as claimed in claim 1, characterized in that, as themaximum value of current density, in a seawater application, a currentdensity of an order of 2.5 A per sq.m is used, and/or a voltage of anorder of 1 V to 100 V is used as the maximal value of the voltage.
 12. Amethod as claimed in claim 1, characterized in that, in industrialprocesses, as the maximal value of current density, a current density ofan order of 10 A per sq. m is used, and/or a voltage of an order of 100V is used as the maximal value of the voltage.
 13. A method ofinhibiting the growth of organisms on a surface of a structure submergedin a liquid, said method comprising the steps of:(a) connecting saidstructure to a source of direct current, so that said structure becomesa cathode; (b) connecting an anode to said source of direct current,said anode not being in physical contact with said structure; (c)controlling said source of said direct current so that said currentchanges in density or voltage, whereby the pH of said liquid on saidsurface of said structure varies.
 14. The method as claimed in claim 13,further comprising the step of coating said structure with anelectrically conductive material.